(a) Field of the Invention
The present invention relates to a method of generating and receiving a packet in a low energy critical infrastructure monitoring (LECIM) system.
(b) Description of the Related Art
FIG. 1 is a diagram illustrating an LECIM wireless network.
Referring to FIG. 1, the LECIM wireless network includes a mains-powered coordinator and a plurality of battery-powered endpoint devices.
The mains-powered coordinator is connected to another mains-powered coordinator and manages a plurality of battery-powered endpoint devices.
The plurality of battery-powered endpoint devices are connected to the mains-powered coordinator. Even if a plurality of battery-powered endpoint devices are not managed by manpower, the plurality of battery-powered endpoint devices should operate for several years and thus the plurality of battery-powered endpoint devices should have excellent low power characteristics. Further, because a communication radius is several hundreds m to several km, the plurality of battery-powered endpoint devices should maintain a reliable communication quality even in a wireless environment having a serious path loss.
A frequency band to operate the LECIM wireless device may be locally different, and the LECIM wireless device is worldwidely operated in a frequency band of 868-870 MHz, 902-928 MHz, and 2400-2483.5 MHz. In the Republic of Korea, 917-923.5 MHz bands that can radiate transmission power of maximum 10 dBm are used as an operation frequency for the LECIM wireless device.
FIG. 2 is a graph illustrating a path loss according to a distance in an LECIM wireless network.
In FIG. 2, in a 900 MHz operation band, an Okumura-Hata model was selected as an LECIM channel model according to a distance. In a 900 MHz band, when a coordinator was positioned at a height of 30 m and an endpoint device was positioned at a height of 2 m, a path loss was displayed according to urban_mid/small, urban_large, suburban, and rural environments.
Referring to FIG. 2, in urban_large, in a distance of 1 km, a path loss was about 125 dB. In urban_large, a path loss of about 30 dB occurs further than in free space, and an urban_large environment is a poorer condition than a channel environment of existing other wireless systems. Therefore, even if a mains-powered coordinator having excellent transmitting/receiving characteristics has a beam-forming gain and an antenna diversity gain, a very weak signal may be received in a receiving terminal
TABLE 1Received power calculated in 900 MHz band and urban_large channel environmentsNotesChannel Model ParametersFrequency (MHz)900Valid Range 150~2400 MHzCollector Antenna Height (m)30Hata Valid Range 30-200 m, including terrain.Erceg Valid Range 10-80 m, including terrain.endpoint Antenna Height (m)2Hata Valid Range 1-10 m, Erceg Fixed to 2 m.Distance (km)1Valid Range 1-20 kmDownlink Path Loss CalculationCollector Tx Power (dBm)10Subject to Tx power regulationsCollector Tx Antenna Gain (dBi)6Subject to Tx power regulationsPath Loss (dB)−125.40Must reference the right path loss from theHata or Erceg worksheetShadowing Margin (dB)−12To buffer against variable shadowing lossPenetration Loss (dB)0For underground vaults, etc.Endpoint Rx Antenna Gain (dBi)2If using same antenna for Tx,must be same as in Uplink tableEndpoint interference (dB)1Rise over Thermal interferenceRx power at Endpoint (dBm)−118.40Compare against Rx sensitivityUplink Path Loss CalculationEndpoint Tx Power (dBm)10Subject to Tx power regulations,Can be different from CollectorEndpoint Tx Antenna Gain (dBi)2Subject to Tx power regulationsPenetration Loss (dB)0For underground vaults, etc.Path Loss (dB)−125.40Same as DownlinkShadowing Margin (dB)−12Same as DownlinkCollector Rx Antenna Gain (dBi)6If using same antenna for Tx,must be same as in Downlink tableCollector interference (dB)2Rise over Thermal interferenceRx power at Collector (dBm)−117.40Compare against Rx sensitivity
Table 1 represents received power that is calculated in 900 MHz band and urban_large channel environments. In Table 1, in urban_large of the Republic of Korea, when transmitting a signal with 10 dBm, a weak signal of about −117 dBm to −118 dBm was received in the receiving terminal.
Intensity of a signal may be changed according to a channel bandwidth, but when such a weak signal is received, if a signal to noise ratio (SNR) is measured in the receiving antenna, the SNR may be 0 dB or less and thus it may be difficult to restore a received signal. Accordingly, in order to restore a weak received signal, by introducing a narrow band physical layer (PHY) that may lower a noise level in the receiving terminal, using a high performance modulation method, and using a method such as channel coding, spreading, and packet repetition or re-transmission, an SNR should be increased.
However, as described above, in order to restore a received signal, when a high performance of transmitting/receiving technique is used, there is a problem that it is difficult to maintain low power characteristics.